A thermal head is well known that includes a bulging glaze layer formed on an insulating substrate in an upheaved manner like a convex lens, and a heating resistor layer formed on the bulging glaze layer. The bulging glaze layer serves to facilitate the contact of transfer ribbon or thermosensitive recording paper with the heating resistor layer, while also serving to improve the heat-reserving performance at heating portions. A thermal head having such an arrangement is disclosed in Japanese Utility Model Publication No. 7-23265 for example.
For convenience of explanation, the specific arrangement of the thermal head disclosed in the above publication will be described with reference to FIG. 8 of the accompanying drawings of the application. As shown in the figure, in the well-known thermal head, a bulging glaze layer 22 made of amorphous glass is formed on a ceramic insulating substrate 21, and an electrode-carrying glaze layer 23 made of crystallized glass is formed to partially overlap an edge portion 22a of the bulging glaze layer 22. Further, a heating resistor layer 25 and an electrode layer 24 are formed on the electrode-carrying glaze layer 23.
With such an arrangement, the electrode-carrying glaze layer 23 is present at the border between the edge portion 22a of the bulging glaze layer 22 and the insulating substrate 21. Thus, the height difference at the border is reduced. Therefore, it is possible to prevent the heating resistor layer 25 and the electrode layer 24, each of which is formed with a small thickness on the electrode-carrying glaze layer, from being cut off or having improper resistance due to the large height difference.
In the above conventional thermal head, the bulging glaze layer 22 is made of amorphous glass, whereas the electrode-carrying glaze layer 23 is made of crystallized glass for the following reason. In forming the electrode-carrying glaze layer 23, a glass paste material for the electrode-carrying glaze layer 23 is printed on the bulging glaze layer 22, and then the printed glass paste is baked. Thus, if the baking temperature for the glass paste is equal to or higher than the baking temperature for the bulging glaze layer 22, the bulging glaze layer 22, which is formed earlier, is unduly softened to undergo deformation, thereby giving rise to inconveniences. For instance, the upheaved portion of the bulging glaze layer 22 may be unduly reduced in height. For purposes of preventing such an inconvenience, conventionally, the electrode-carrying glaze layer 23 is made of a crystallized glass which can be baked at a lower temperature compared to the amorphous glass used for forming the bulging glaze layer 22.
However, with the conventional arrangement, the electrode-carrying glaze layer 23 and the bulging glaze layer 22 are respectively made of a different material. Thus, in forming these two glaze layers 22, 23, it is necessary to prepare two kinds of material and selectively use either material depending on the kind of the glaze layers. Such a procedure is troublesome, and production efficiency remains yet to be improved.
Further, in such a thermal head in general, the surfaces of the heating resistor layer 25 and the electrode layer 24 are covered by an insulating protection layer (not shown) made of a glass material. It is preferable to form the insulating protection layer from an amorphous glass capable of providing a smoother surface than a crystallized glass, since the insulating protection layer is brought into direct contact with a transfer ink ribbon or thermosensitive recording paper. When the insulating protection layer is made of amorphous glass, the materials of the electrode-carrying glaze layer 23 and the insulating protection layer differ in kind. Thus, when the electrode-carrying glaze layer 23 is made of crystallized glass, the number of material replacement becomes still larger, thereby decreasing the production efficiency.
Further, in the conventional thermal head, the electrode-carrying glaze layer 23 is made of crystallized glass, which provides a coarser surface than an amorphous glass. Thus, cutoffs are likely to occur in the heating resistor layer 25 and the electrode layer 24 formed on the surface of the electrode-carrying glaze layer. Thus, the conventional arrangement remains yet to be improved also in view of the prevention of the cutoff in the heating resistor layer 25 and the electrode layer 24 formed on the surface of the electrode-carrying glaze layer 23.